Filter circuit for radio receiving sets



March 1, 1932. D. GRIMES 1,847,759

FILTER CIRCUIT FOR RADIO RECEIVING SETS Filed Nov. 2, 1926 3 Sheets-Sheet 1 INVENTOR.

N; 2 Ifl" ATTORNEY.

March 1, 1932. D. GRIMES 1,847,759

FILTER CIRCUIT FOR RADIO RECEIVING SETS Filed Nov. 2, 1926 3 Sheets-Sheet 2 I N VEN TOR..

IF BY 7 q j lw g I. ATTORNEY.

March 1, 1932. IM s 1,847,759

FILTER CIRCUIT FOR RADIO RECEIVING SETS Filed Nov. 2. 1926 3 Sheets-Sheet 3 INVENTOR.

TORNEY.

Patented Mar. 1, 1932 mire STATES PATENT OFFICE DAVID GRIMES, OF GRASMERE, NEW YORK, ASSIGNOR TO RADIO CORPORATION OF AIJERICA, .A CORPORATION OF DELAWARE FILTER omourr r03 name RECEIVING SETS Application filed November 2, 1926. Serial No. 145,735.

My present invention has for its object to provide certain improvements in circuits for radio receivers which are primarily intended to afford substantially equal amplification of all the various wave lengths used in broadcast transmission and which will provide the necessary sharpness in tuning on each of said various wave lengths to enable them to be easily selected and the possibility of inter ference avoided. Stated in another way my invention contemplates a circuit, or combination of circuits, whereby equal efliciency throughout the broadcast band of radio frequencies is obtained, considered either from the standpoint of selectivity or amplification,

in that in the arrangement proposed the tuning is equally sharp on the lower frequencies (which produce the long waves) and on the higher frequencies. (which produce the. socalled short waves), because I am able to prevent a falling off in the amplification of the longer waves which loss has been heretofore due to loss in the transference of energy of such longer waves in the usual induction transformers.

My invention broadly stated comprehends a method of effecting a circuit coupling between two or more audions or vacuum tubes connected incascade which comprises selectively feeding back a portion of the amplified energy from the output of a second tube to the input of a preceding tube in amounts to substantially equalize the resultant amplification over a band of wave lengths.

To these and other ends, my invention further consists in certain additional improvements all as will be fully set forth in the accompanying specification and particularly pointed out in the claims appended thereto.

In the drawings,

Fig. 1 is a diagram showing in simplified form a schematic circuit illustrating one embodiment of my present invention;

Fig. 1 is a diagram illustrating the relative directions in which the primaryand secondary windings of the transformers are applied in practice. Y I

Fig. 2 is a diagrammaticillustration showing one form of a commercial type of receiving circuit embodying my invention, and

Fig. 3 is a similar view illustrating the invention applied to the so-called inverse duplex receiving circuit.

Similar characters throughout the drawings illustrate similar parts.

My invention is directed particularly to providing a commercial type of circuit for radio receiving sets the success of which depends upon the satisfactory reception of radio signals on all of the various so called broadcast wave lengths which at the present time vary from 200 to 550 meters. The commercial success of such a set also depends upon the ability of the operator to effect selectivity between various stations operating under assigned wave lengths and to obtain a requisite and equal amplification of such wave lengths.

The outstanding point of criticism in many of the receiving sets in common use at the present time arises from the necessity of employing, when a plurality of audions or vacuum tube amplifiers are used, transformers comprising primary and secondary windings, and many inherent defects in such sets are attributable to the transformer action varying'for different wave lengths. It is a wellknown physical fact that on the shorter wave lengths, which are'transmitted by the higher frequencies, the transfer of energy from the primary to the secondary coil of a transformer is much better than it is with the longer wave lengths which are transmitted by the lower frequencies. Conversely stated, when the circuits are tuned for the reception of the longer wave lengths there is a reduction in the amplification of the received signal. In attempts to obviate this difliculty certain circuit connections have been proposed, but in these arrangements the attempts to increase the amplification at the long waves have resulted in broadening the tuning, thus rendering the circuits as a whole less selective in the long wave lengths occupying the upper range of the broadcast band.

In the circuits disclosed in this application,

I have, with full appreciation of the difiiculties above outlined, selected a transformer most eflicient at the shortest'wave length to be received and combined'with the circuits Gil associated therewith, means whereby, as these circuits are tuned in resonance with stations transmitting signals of varying wave lengths, the longer waves will be progressively and increasingly reinforced. For instance, as suming that a wave length of 250 meters shows a slight reduction in amplification, a small portion of the oscillations in step with this wave length emitted from the output circuit of the last amplifying tube is used by feeding it back to the input circuit of a preceding tube to bring the final amplification of this wave length up to the desired point, or that equal to the amplification obtained for a wave of 200 meters. Again, if the circuits are tuned for a wave length of, say, 300 meters, an increasing proportion of the corresponding wave length amplification or oscillation occurring in the output circuit of the last tube would be employed as the increment for bringing this wave length to a point of amplification equivalent to that of the 200 meter wave reception. At the extreme wave length used in broadcasting, or 550 meters, a considerably larger reinforcing .current is thus utilized to compensate for the extensive reduction in the normal amplification which occurs at this frequency compared with the 200 meter efficiency.

This increased amplification obtained by reinforcement is equivalent to obtaining amplification by a reduced resistance. This increase in amplification it will be seen can therefore be obtained from a reinforcement of the circuit which becomes increasin ly effective as the wave lengths are increased.

This reinforcing of the original Wave acts on the signal in the same manner as if the resistance in the circuit had been counterbalanced proportionately for each increase in wave length, thus maintaining the sharpness of the resonance curve substantially equal on. all wave lengths.

In the simplified diagrammatic illustration shown in Fig. 1, the receiving circuit is composed of the thermionic amplifiers A and B, representing the first and second radio frequency stages, and a third tube C comprising the usual detector. The weak radio broadcast signals are received on the antenna 1 which'is preferably connected with a tuned primary coil 2 from which a tap leads to ground. Associated with this coil is a secondary winding 8 located in the input of tube A by connecting one end to the grid 4 and its other extremity by a conductor 5 to the filament 6. Means are provided for tuningthis circuit to the desired frequency of the incoming antenna wave by means of avariable condenser 7 bridged between the grid 4 and the conductor '5. Theoutput' circuit of the .tube A consists of the plate 8, the primary winding 9 and the battery 10, the negative pole ofwhich is connected to the filament 6.

The tubes Aand B are coupled through transformer which also comprises the sec ondary winding 12, associated with the primary winding 9, and located in the input circuit of the second tube B comprising the grid 13 and filament 14. This circuit is tuned by the variable condenser 15 connected in bridge between said grid and filament. This input circuit, it will be understood, is tuned in resonance with the output circuit of the tube A. The output circuit of tube B comprises plate 16, primary winding 17, of the second radio frequency transformer, and the battery 18, the negative pole of which is connected to the filament 14. The secondary coil 19 of the last mentioned transformer is connected to the inputcircuit of the detector tube C which comprises the grid 20 and the filament 21'. Tuning of this circuit is likewise accomplished by providing across the terminals of the last mentioned coil a variable condenser 22. If desired, the signal detecting device, such as a telephone receiver 23, can be con nected directly in the output circuit of the tube C by connecting'the plate 24 thereof withone side of the receiver circuit by means of the conductor 25 and completing the circuit from the receiver by another conductor leading to the battery 26, the negative pole of which is in turn connected by conductor 26 to the filament 21. When a telephone receiver is employed in such an arrangement it is of high resistance and is bridgec by a by-passcondenser 28 of small capacity.

Thefilament of the tube 0 is lighted by current from battery 29 and the filaments of tubes B and A are lighted by batteries 29 and 29", the negative terminals of which are, for convenience in further description of the present circuit, illustrated as being grounded.

For the purpose of simplification I have shown in Fig. 1 a common lead 30 connecting the output circuit of the last radio :lrequcnc"; amplifier tube 13 with the input of the first tube A, and located therein, indicated at 31, is a radio frequency filter coil of approximately .7 5 millihenries and 1000 ohms resistance. It will also be observed that in the output circuit of tube B between the negative pole of the battery 18 and the filament 141 have placed a high resistance 82, of approximately 25,000 ohms, which affords a return from the battery 18 to the filament of tube B. this resistance beingsubstantially equivalent to the internal resistance of the tube 13 itself.

Located in the input circuit of tube A between the filament 6 and the lead 30 there is a very much higher resistance 33 of approximately one megohm. The object of this resistance is to permit the grid 4 of tube A to be maintained at the same direct current potential as the negative end of filament 6 of tube A.

In conjunction with the aforementioned radio frequency filter coil 31, I place between its opposite ends and the lead connected to the negative end of filament 14 of tube 13 l This nde ser 35 can e of small r 3 is finally pa e bac to. the filam n i of tube B through the filter condenser 85.

capaci y tha de ser .34 because only v ry slight portions, if any of the. 200-mcter currents will pass through the filter coil, and there is no tendency for such currents to pass beyond this point With the size mentioned of capacity 35, because this will effectively pass them'to. filament 14;. This filter condenser 33 also functions as a return from the first tuned secondary coil 3. to the filament 6 of tube A,

: through the ground connections of batteries 29 and 29 The apparatus Just described functions when the circuits described are tuned to the proper resonance for so-called long wave length reception m a slightly different manncr'from that previously described, in that the wavelength energy encounters increasmg difficulty 1n passing through condenser 34 findin at the same t me a more read r path through the filter coil 31. These are welibknown functions ofcondensers and coils in general, as a condenser offers more impedance to low frequencies than high frequencies and a coil performs in the reverse manner. The long wave currents passing with more case through the filter coil 31 are forced on through the tuning coil 3 to. 4 of tube A because of the high resistance 33 and the high reactance of condenser 35 at the long wave lengths. It can be seen therefore that a portion of the long waves which pass the filter coil 31 being of the same frequency as the incoming currentsin the tuned circuit 3'? reinforce the original. weak incoming current transmitted from the antenna to said grid 4. This reinforcing effect increases the original wave strength and counte-rbalances the effective resistance in the various tuning circuits so that not only an increased amplification is obtained but also sharper tuning is efiiected.

It can be seen from the action of the filter condensers 3i+3f and coil 31 that the reinforcing becomes greater the longer the wave length and becomes less the shorter the Wave length. The reinforcing therefore compensates for the decreasedltransfer efliciency be tween the, transformer coils 9-12 which. is well known to. be less than half as effective at 55d meters as zit-200; meters." The capacitance, indu an a d resis a ce in this filter combination may be varied from the values heretofore stated and it will be understood that the object is to so adjust them that the increase in reinforcing effect is proportional to the decrease in transfer effect between the transformer coils 912, thus giving an over all amplifying effect by a summation of the transferred energy plus the reinforced energy on every broadcast wave length.

In other words, the principle of the system herein described is to maintain what m y be called cons ant and equal intensity value of the received signal throughout the circuit, the difference between the actual intensity of the progressively increasing wave lengths being progressively boosted by an increment which will give the intensity con stant above described.

The proportion of the reinforcing currents seeping through the filter coil 31 must have an additive effect upon the received signal'when reaching the input of the tube A, and in order tocreate this effect it will be appreciated that it is necessary that the proper phase connections be maintained between the terminals of the primary and secondary transformer coils 912 and the re spective elements of the tubes A and B to which they are connected. In Fig. 1 I have illustrated the proper way to make these connections.

For instance, these two coils 9 and 12 are wound in the same direction of rotation and connected to the tube elements so that the m0 In Fig. 2 I have shown diagrammatically ho a commercial form of radio receiving circuit embodying the principles above outlined. This layout comprises the radio freg tubes A and B and the detector C embodying grids 4 13 and 20,; the filaments 6 1%? and 21, and the plates 8 16 and 24. The antenna 1" has the tuned primary coil 2 associated with the secondary coil 3 which is tuned to the frequency of the desired incoming signal by the variable conquency amplifyin denser 7 and is connected to the grid 4 The tubes A and B are coupled by means of a transformer, the primary 9" of which is connected to theplate 8 and the secondary in; 12 connected to the grid 13 being tuned by the variable condenser 15 A similar transformer connects tubes B and C by having its primary winding 17 connected to. the plate 16. and its secondary 19 to grid 20 and tuned by the variable condenser 229-.

The several filaments 6 14 and 21 are lighted from a six volt battery by connecting their positive sides by a conductor 51 and their negative sides by a conductor 52, which conductors are connected respectively to the positive and negative terminals of said battery; a filament lighting circuit includes a rhcostat 52 by means of which the filament temperature may be controlled.

The input circuit of tube A is completed by the conductor 500 which is joined to the battery lead 52 of filament 6, preferably at a point between the battery 50 and the rheostat 52, making the connection at the point shown has the advantage of giving a slight negative bias on the grid circuit due to the slight drop in voltage in said rheostat. In the conductor 00, I locate the resistance 33 The output circuits of the radio frequency amplifying tubes A and B are completed through the usual 90 volt plate circuit battery 53, by connecting the negative pole of this battery to the common positive filament lead 51 and its positive pole to a conductor 55 leading to the end of the primary coil 9 of the first coupling transformer and by a branch 56 leading to the corresponding coil 17*" of the second coupling transformer. The battery is divided into a 22% volt section 54, and the detector tube C has its plate 24 connected through the signal receiving device 23 by the conductor 57 to the 22 volt point of the plate circuit battery. Across this circuit is a bypass condenser 58.

In this circuit arrangement I utilize the filtering elements before described, comprising a series inductance and shunt condensers at opposite sides thereof, which provide an effective path for the transfer of a portion of the amplified long wave ener y emitted from the output of the second tube B to the input of the first tube A via the use of a condenser of lar 'e capacity, such as one microfarad condenser, 59. This condenser is not shown in Fig. 1 because in the circuits therein illustrated a separate source of battery, or independent source of energy, is employed for each tube. However, in commercial receiving sets in which a common so called B battery is employed, it becomes necessary to provide a means in the reinforcement conductor leading from one tube to the preceding tube to keep the high po tential plate current of said first tube out of the grid circuit of said preceding tube.

In the conductor 56 I interpose the radio frequency coil 31" and the high resistance 32 Of the two condensers of the filter system that which is of the higher capacity indicated by 34?, connects the conductor 56 with the wire 52, thus affording a path for high frequency currents to the filament 14- at a point in front of the filter coil 31 The second condenser 35 of the combination is connected in bridge between the input or grid circuit conductor 500 and the filament lead 52 in which position it is located with reference to the condenser 34? at the opposite side of the filter coil 31*;

The values given for the condensers and coil of the filter system are the same as those mentioned in the description of the circuit of Fig. 1 and their operation is the same. It is understood that any tendency for current on the lowest wave length to escape from the output of tube B into the input of tube A is substantially blocked by the filter coil 31 being initially passed back to filament 14 through the condenser 34*. Should a portion of the current at this wave length pass through the coil 31 it will find a path to the filament 14* through condensers 59 and 35 hen the apparatus is operating on the longer waves, these finding a more difficult path through condenser 34 and a more ready path through the filter coil 31 pass with virtual freedom through the condenser 59 into the input circuit of tube A and due the high reactance of condenser 35 at this wave length and high resistance 33 are directed to and become a potential on the grid 4 which being in step with the received sig nal, as determined by the phase connections of the transformer 9 -12 reinforces said signal current to a point where the grid control of the electronic stream flowing from the filament 6 to the plate 8 becomes very effective.

The adaptability of my invention to various types of radio receiving systems is further illustrated in Fig. 3 in which I have shown an inverse duplex hookup of the general character shown in Patents No. 1,517,- 057 and 1,517,058 granted to me November 25th, 1924, The vacuum tubes G, H and I shown in this illustration represent in the order given the first and. second stages of radio frequency amplification and the detector. The circuit connections as will be described are such that in the tube H the first stage of audio amplification is obtained and in tube G the second stage of audio amplificat-ion occurs.

60 indicates the antenna circuit having the tuned primary coil 61 with which is associated the secondary coil 61 connected to the grid 9 and the filament 9, these wiring connections including the variable condenser 62, by means of which the input circuit of tube G is tuned to the wave length of the desired incoming signal received from the antenna circuit.

The output of tube G comprises the plate element the circuit from which comprises the primary winding of a radio frequency transformer 65 one end of which is connected by conductor 66 to the positive terminal of the plate battery 67, the other pole of which is connected to the filament lead 75. The circuit is coupled to the input of tube H by connecting the secondary coil of the transformer to the grid it and coupled there-across is a tuning condenser 68. The plate circuit of this tube leads from plate 7& through the primary winding of a second radio transformer 69 to battery (57 over conductor 70.

The active tube I has an input circuit em-- bracing the secondary coil of the last mentioned transformer, the ends of which are connected to grid 2' and filament z" and to the variable condenser 71. The output of this circuit leading from the plate 2' is represented by the conductor 72 which extends to an intermediate point on the battery 67.

The filaments of the several tubes are lighted from the common battery 63 by connecting one end of each to the negative lead 64 and their other extremities to a common return 7 5. In one of these leads I include an adjustable resistance element 64 for controlling their temperature.

Ext-ending between the input circuits of the tubes G and H and the negative ends of their filaments are circuit connections containing by-pass condensers 135 and 7L3, respectively, and similar connections are made between the positive sides of the filaments of all the tubes and their respective output circuits which include similar condensers 9*, 134 and i These several condensers are all of small capacity and arranged to pass high frequency current and impede the passing of audio frequency currents.

The circuit shown in Fig. 3 is of the inverse duplex type, hence the output of audio frequency currents from the detector tube I are fed into the input of tube H, and upon being amplified therein are passed into tube G for final amplification and delivered into the signal receiving device which may be a telephone or loud speaker 7 7 To this end the coupling between the output circuit of tube I and the input of tube H is effected through an audio transformer 78, having an iron core,

by connecting its primary winding in the conductor T 2 and its secondary in the conductor 79, one end of which leads to the grid h of tube H, its other extremity being connected to one pole of a third, or separate battery 80, the other pole of which is connected to the negative filament lead 64. A similar connection 81 leads from the circuit of grid 9 of tube G to the separate battery 80.

In place of another audio transformer for effecting the audio connections between the output of tube H and the input of tube G, I utilize the elements of the filter combination before described in the following manner.

In the conductor it place the radio frequency filter coil 131 (corresponding to coil 31 of Fig. 1 and 31 of Fig. Between this coil and its point of connection to the battery 67 I place the high resistance 132. The higher capacity condenser of the pair of condensers associated in action with said coil, indicated m'entg, as indicatedat135.

A- path for both-radio and audio frequency currents iszprovided between the last stage of" amp-lificationand the input of the first tube by placinga large capacity. condenser 159 which-sis connected at one of its sidesat a point beyond the output end-of coil 131 and at its other side connected to conductor 81. This condenser 159 is placedin the position shown merely toprcvent thehigh plate current potential'of battery 67*(present in conductor: 7,0)from reaching grijdg of tube G.

Inserted in the conductor 81 between thev condenser 15921l1d. the battery 80. isthe high resistance element 133 of the filter combinaion. The operation; ofthe foregoingcircuit will beyobvious. To state it briefly with respect to the features of my present invention it will-be assumed first that a short Wave frequency is being-considered. The high frenency oscillations pass readily through the fixedcondenser 13a to filament 72. while at thesame time-the same'oscillations, being of hi h frequency, encounter 3 considerable diliiculty in attempting to pass throughthe filter coil 131 'hich has a real choking'action at thesefrequencies; thiscurrent as mayseep through this coil is readily passed to filament gt through the condenser 135 and arethus prevented from reachingthe as a reinforcing action at thiswave lengtliis-n'ot desired.

As thelargenwave lengths are tuned in on the amplifier the lower frequencies, correspondlngto these longer waves,find increasing difiiculty in returning to filament h" through co'nden-ser l34t and at the same time pass more readily through the filtercoil 131, the-choking effect of which becomes nounced the lower the frequency. Thus, an appreciableamountofthe long, wave oscillations pass back through-the choke 131 to the grid ofthefirst-radio frequency tube G reinforcing'theori inal long wave signal to such an extent as to compensate for they decreased couplingefliciency' of the; tuned transformer 65. The fixed condenser 135 which passed the high frequency seepage currents through filter coill'131-to filament g5 ofiers considerable impedenceto the low frequency oscillations which have passed through for reinforcing purposes.

What I claim is 1. A high frequency signal system comprising a plurality of coupled audion circuits and a fixed feed-back circuit from the output of one audion circuit to the input of the preceding audion circuit, said feed-back circuit affording an easier path for low radio Such small amount of,

grid 9 ofthc fiist radio less protween any two of said amplifiers,

frequency currents than forhigh radio frequency currents to substantially equalize the transfer of energy over a band of wave lengths. y i

2. A. Wave amplifying system comprising a plurality of coupled amplifier circuits, a

feed-back circuit from. the output of one 7 amplifier to the input of a preceding amplifier, and a wave filter in said feed-back circuit, said filter being designed to compensate for the d'scrimination in amplification of Waves over a band of Wave lengths.

3. In a wave amplifying system, a plurality of coupled amplifiers including at least one space discharge device and parallel paths for the transmission of energy between two of said amplifiers, one of said paths having electrical characteristics which introduce variations in the efficiency of transmission for different frequencies over a given.

range, said other path comprising means for compensating for such variation in transmission efiiciency to produce substantially uniform transmission over the entire range and oneof said paths providing energy transfer across and externally of a space discharge device;

4. In a Wave amplifying system, a plurality of coupled amplifiers and parallel paths for the transmission of energy beone of said paths having electrical characteristics which introduce variations in the efficiency of transmission for different frequencies over a given range, and said other path comprising means for compensating for such variation in transmission efiiciency' to produce uniform transmission over the entire range, said paths being designed to transmit energy in opposite directions between said two amplifiers.

5. A Wave amplifying system comprising a' plurality of coupled audion amplifier cir-.

cuits, a feed-back circuit from the output of one amplifier to the input of a preceding amplifier, and a Wave filter in said feed-back circuit comprising a series inductance and shunt condensers connected across said circuit on each side of said inductance.

6. In an audion amplifier having tuning means for selective amplification of currents of varying frequency, a fixed feed-back circuit including a Wave filter, conductively connectedto the input and output circuits of the amplifier, to substantially equalize the transfer of energy and the sharpness of tuning over a band of Wave lengths.

DAVID GRIMES. 

